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  4. The conjugate base of H2PO4- is

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Q. The conjugate base of $ {{H}_{2}}PO_{4}^{-} $ is

JamiaJamia 2007

Solution:

$ {{H}_{3}}P{{O}_{4}} $ is a tribasic acid, thus ionising in three steps: I. $ {{H}_{3}}P{{O}_{4}}{{H}^{+}}+{{H}_{2}}PO_{4}^{-} $ II. $ {{H}_{2}}PO_{4}^{-}{{H}^{+}}+HPO_{4}^{2-} $ III. $ HPO_{4}^{2-}{{H}^{+}}+PO_{4}^{3-} $ Conjugate base is formed when an acid loses its proton. Thus $ HPO_{4}^{2-} $ is the conjugate base of $ {{H}_{2}}PO_{4}^{2-} $ (which is an acid in step II, but is the conjugate base of $ {{H}_{3}}P{{O}_{4}} $ in step I).